Solving linear equations and inequalities: Linear inequalities in one unknown
Reduction to a linear inequality
In some cases, you can reduce complicated inequalities to linear inequalities.
We note first that division by zero is not allowed and that for this reason \(7x+8\) may not be equal to zero and that therefore \(x=-{{8}\over{7}}\) is not a solution.
We now distinguish two cases, namely \(7x+8>0\) and \(7x+8<0\).
In both cases we multiply the inequality on both sides by \(7x+8\) because we then get a linear inequality, for which we know there is a solution method.
Suppose \(7x+8>0\), i.e. \(x> -{{8}\over{7}}\). Then we get \(3<5(7x+8)\).
When we move everything with \(x\) to the left and all constant terms to the right, we get \(-35x<37\).
Then, dvision by the coefficient of \(x\)gives \(x > -{{37}\over{35}}\).
So we have the following system of inequalities: \(x> -{{8}\over{7}}\,\wedge\; x > -{{37}\over{35}}\)
and this simplifies to \(x\gt-{{37}\over{35}}\).
Suppose \(7x+8<0\), i.e. \(x< -{{8}\over{7}}\). Then we get \(3>5(7x+8)\).
When we move everything with \(x\) to the left and all constant terms to the right, we get \(-35x>37\).
Then, division by the coefficient of \(x\) gives \(x < -{{37}\over{35}}\).
So we have the following system of inequalities: \(x< -{{8}\over{7}}\,\wedge\; x < -{{37}\over{35}}\)
and this simplifies to \(x\lt -{{8}\over{7}}\).
The solution of the original inequality is \(x\lt -{{8}\over{7}}\;\vee\;x\gt-{{37}\over{35}}\).